This IP core has read and write AXI-Stream FIFOs, the contents of which can be accessed from the AXI4 memory-mapped interface. This is useful for transferring data from a processor into the FPGA fabric. The driver creates a character device that can be read/written to with standard open/read/write/close.
See Xilinx PG080 document for IP details.
The driver currently supports only store-forward mode with a 32-bit AXI4 Lite interface. DOES NOT support:
- cut-through mode
- AXI4 (non-lite)
You should find a character device in /dev (e.g. /dev/axis_fifo_########) for each AXI-Stream fifo you create in your hardware. The device can be read and written to like a normal file.
To quickly test things, you can write a packet to your fifo from the command line with
echo -n -e '\xDE\xAD\xBE\xEF' > /dev/axis_fifo<#>
and read from it with
cat /dev/axis_fifo<#> | hexdump -C.
When you write data to the fifo, the block of data is written as a packet as dictated by the length passed to write. When you call read(), a single packet is returned regardless of how many bytes were requested.
For example:
// assuming the fifo is configured in loopback mode in hardware
// e.g. axis tx interface feeds back into axis rx interface
// note that in this case a single file descriptor could be opened for both read and write,
// but separate read/write descriptors are used for illustrative purposes
int f_wr = open("/dev/axis_fifo_43c00000", O_WRONLY);
int f_rd = open("/dev/axis_fifo_43c00000", O_RDONLY);
// writes a packet of 12 bytes (3 words)
write(f_wr, &write_buff, 12);
// returns 12 bytes
ssize_t bytes_read = read(f_rd, &read_buff, 100);
close(f_wr);
close(f_rd);By default, read() and write() will block for one second before timing out. You can change this behavior by loading the module with command line arguments "read_timeout" and "write_timeout" (in milliseconds):
insmod /lib/modules/4.9.0-xilinx-v2017.4/extra/axis-fifo.ko read_timeout=100 write_timeout=5000
You can set "read_timeout" / "write_timeout" to a negative value if you want read() / write() to block indefinitely:
insmod /lib/modules/4.9.0-xilinx-v2017.4/extra/axis-fifo.ko read_timeout=-1 write_timeout=-1
You can also make non-blocking reads/writes by opening the file with the O_NONBLOCK flag set which will cause read() and write() to return immediately:
int f = open("/dev/axis_fifo_43c00000", O_RDWR | O_NONBLOCK);See fifo-test.c for more detailed usage code and to test functionality/throughput of your FIFO.
See fifo-test-eth.c provided for simple echo server demo using the fifo.
See axis-fifo.txt to see example device tree entry.
The AXI-Stream protocol requires the TKEEP flag to be enabled in order to process byte width transactions over a 32-bit bus. When the TKEEP flag is NOT used then the FIFO is only able to process on word boundaries.
TKEEP must be both enabled on the IP Core and the device-tree by use of the has-axis-tkeep = <1> entry in the device tree. When it is NOT enabled byte width read/write requests will present an error and not be written to the core.
fifo-test.c supports testing of both modes.
The poll mechanism works off of the assumption that the user has an idea for a minimum and maximum packet size with respect to the AXI-Stream framing. To support this there are two entries in the device tree,
xlnx,rx-min-pkt-size = <255>; /* 1020 bytes */
xlnx,tx-max-pkt-size = <257>; /* 1028 bytes */
When the Transmit Data FIFO Vacancy register (TDFV) is greater than tx-max-pkt-size poll will return POLLOUT.
When the Receive Data FIFO Occupancy register (RDFO) is greater than rx-min-pkt-size poll will return POLLIN.
Example code using poll is provided in fifo-test.c
NOTE : tx-max-pkt-size and rx-min-pkt-size are defined as WORDS not BYTES.
You can access the IP registers directly if you wish using sysfs. They are located in
/sys/class/axis_fifo/axis_fifo_########/ip_registers/
For example, you can read the RDFO with
cat /sys/class/axis_fifo/axis_fifo_########/ip_registers/rdfo
or write to the fifo/TDFD with
echo 0xdeadbeef > /sys/class/axis_fifo/axis_fifo_########/ip_registers/tdfd.
The driver supports a number of ioctls for setting,
- read/write to any register
- reseting the core
- set/get tx-max-pkt-size for poll()
- set/get rx-min-pkt-size for poll()
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